http://afe.easia.columbia.edu/webcourse/key_points/kp_10.htmJapan, which had isolated itself from international politics in the Tokugawa period (1600-1868), enters an international system of the late 1800s where imperialism dominates. Japan rapidly becomes a major participant in this international system and seeks particular imperialist privileges with its East Asian neighbors, China and Korea.
By 1910, Japan annexes Korea as a colony and takes control over indigenous Korean modernization efforts. In 1931, Japan takes control of Manchuria and establishes the puppet state of "Manchukuo"; in 1937, Japan invades the rest of China.
Look the USA does too:Following the successful opening of Japan to trade and diplomacy with the West in 1854 through the "gunboat diplomacy" of Commodore Perry of the US Navy, the British, the French, and the Americans all attempted to open Korea in a similar fashion. Korea, however, refused to comply to Western demands, and engaged in naval skirmishes with the French and the Americans in the 1860s and early 1870s.
In the end, the country was forced to open up not by the West, but by Japan itself. The 1876 Treaty of Kanghwa between Japan and Korea, named after the island off the west coast of Korea where it was signed, was a classic "unequal treaty" of the kind Western powers were imposing on Asian countries, including China and Japan, in the nineteenth century. The treaty gave Japan special trading rights and other privileges in Korea that were not reciprocated for Koreans in Japan. The United States and major European countries soon followed with their own treaties of trade and diplomacy with Korea.
By the end of the nineteenth century, rivalry over Korea led to war between Japan and China (1894-95) and, ten years later, between Japan and Russia (1904-5). Japan won both wars, and in 1910 Japan annexed Korea as a colony, ending the Choson dynasty after more than 500 years of independent rule.